Mercury recovery from sediment

ABSTRACT

A METHOD IS PROVIDED FOR RECOVERING MERCURY FROM SEDIMENT AT THE BOTTOM OF A BODY OF WATER BY OVERLAYING THE SEDIMENT WITH A LAYER OF METAL CONTAINING AT LEAST ONE OF ALUMINUM, MAGENSIUM AND ZINC. PREFERABLY, THE METAL IS IN SHEET FORM FOR MORE EFFICIENT INTERCEPTION OF MERCURY VAPOR RISING FROM THE SEDIMENT. AFTER REMOVAL OF THE METAL FROM THE WATER BODY, THE MERCURY, WHICH FORMS AN AMALGAM ON THE SURFACE OF THE METAL, MAY BE RECOVERED BY DISTILLATION FROM THE METAL SURFACE.

United States Patent 3,679,396 MERCURY RECOVERY FROM SEDIMENT Vernon A. Stenger, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich. No Drawing. Filed Ian. 14, 1971, Ser. No. 106,559 Int. Cl. C22b 7/00, 43/00; E02d 29/00 US. Cl. 75-81 6 Claims ABSTRACT OF THE DISCLOSURE A method is provided for recovering mercury from sediment at the bottom of a body of water by overlaying the sediment with a layer of metal containing at least one of aluminum, magnesium and zinc. Preferably, the metal is in sheet form for more efiicient interception of mercury vapor rising from the sediment. After removal of the metal from the water body, the mercury, which forms an amalgam on the surface of the metal, may be recovered by distillation from the metal surface.

THE SPECIFICATION It has been recently discovered that metallic mercury discharged to water bodies, such as rivers and lakes, is gradually transformed into soluble mercury compounds. These compounds become dispersed in the water and ultimately concentrated in marine animal life. Accordingly, there is a need for methods to recover metallic mercury from the bottom of rivers and lakes. Furthermore, there is a need to recover mercury from sludge ponds or lagoons in which industrial waste containing mercury have been impounded.

An object of the instant invention is to provide such a method. A further object is to porvide a method for achieving mercury decontamination of the sediments and ultimate recovery of the mercury as a purified reusable product.

DESCRIPTION OF THE INVENTION In one embodiment, the instant invention comprises overlaying sediment at the bottom of a body of water containing metallic mercury with a layer of a metal containing a mercury amalgamating content of at least one of aluminum, magnesium and zinc. The metal layer overlies the sediment in the sense that it is in relatively close proximity to the sediment from which mercury recovery is desired. Most conveniently, the metal layer will rest directly upon the surface of the sedment at the bottom of the water body but it may be positioned either directly above and somewhat apart from the sediment surface, or even somewhat below the surface of the sediment. It is not essential to the invention that the sediment be of any particular analysis. Bottom sediments are known to vary widely in their consistency and composition. The instant invention is applicable to any form of sediment containing metallic mercury regardless of whether it is essentially organic or inorganic. Inorganic sediments are usually siliceous, clayey, or calcareous in their essential composition.

It is desirable that the metal layer overlay the sediment in an essentially continuous fashion for eflicient recovery of the mercury. Such a layer may take the form of particular or sheet-like materials distributed substantially uniformly over the surface of the sediment. For convenience of retrieval, it is preferred that the layer of metal be in the form of sheets, which may be inter linked to provide shingle or chain-like continuous coverage.

As utilized in the practice of the invention, the amalgamating metal layer may be provided as the elemental active metal but usually it will be in the form of an alloy.

Suitable metals include, for example, the interalloys of aluminum, magnesium and zinc and alloys of these active metals with conventional alloying constituents such iron, silicon, manganese, lead, copper, nickel and titanium. The amount of an alloying constituent need not be controlled so long as the resulting alloy has a mercury amalgamating content of the active metal. Usually the active metal will be present as an external phase within the grain structure of the alloy. That is, the desired metal will not be totally in solution or occluded within an inoperative phase. Whenever the metal to be used has a substantial oxide coating, this should be removed as by washing with a dilute protonic acid, e.g., hydrochloric acid, for most effective results.

The ultimate, and only essential test of a suitable alloy, is that the metal form a surface amalgam with metallic mercury. This fact can be readily ascertained as by dipping the metal in mercury and determining whether the surface is wetted with metallic mercury on removal of the metal.

In a preferred embodiment, the active metal is employed as a cladding or galvanic coating on a substrate such as an iron based sheet metal. This provides an essentially continuous thin layer of the desired metal over a relatively more economicaly supporting substrate. Galvanized sheet metal is a particularly convenient and readily available form of a useful metal.

The length of time during which the sediment should be overlain with, or contacted, by the metal layer will vary according to the amount of mercury to be recovered and the efliciency of recovery desired. Some mercury will be recovered in a few days but more often contact periods of several months may occur before suflicient mercury recovery has been achieved to justify retriveal and, if desired, replacement of the metal layer. Although the amalgam formed on the metal surface can continue to accumulate mercury for extended periods of time, it is desirable, in order to minimize secondary diffusion and oxidation of the amalgam, periodically to remove the metal layer from the water body and strip accumulated mercury from its surface.

Stripping is conveniently accomplished by placing the retrieved mercury amalgamated aluminum, magnesium or zinc metal layer in a heated zone and recovering the vapors emitted therefrom to obtain a mercury distillate. Suitable distillation temperatures will vary depending upon the type of amalgam that has been formed. For zinc amalgams, the distillation of the mercury from the surface of the metal can be achieved at temperatures above about C. at a pressure reduced to below about 0.5 mm. Hg. Somewhat higher minimum temperatures are more desirable in the instance of separation of mercury from aluminum and magnesium. In the latter instances, it is desirable to conduct the distillation in the presence of an inert atmosphere such as argon to minimize the formation of the metallic oxides or nitrides.

The following examples will further illustrate the practice of the instant invention.

EXAMPLE 1 A sludge containing about 600 parts per million by weight of mercury was obtained from the bottom of a mercury contaminated pond. Seven pellets of particulate zinc weighing about 0.8 gram each were washed with dilute hydrochloric acid (1:10), rinsed and air dried. Six of the pellets were then mixed in a flask with 10.2 grams of the sludge and about 30 milliliters of water. This quantity of sludge contained about 6100 micrograms of mercury. The flask was allowed to stand at ambient temperature for an extended period. At certain intervals, one pellet was removed from the flask, rinsed with water, and analyzed for associated mercury. The results atomic absorbance I method. Results are sigpifi cant to of these tests are set forth in the follow g table A. i

- TABLE A Mercury Exposure, found, days micrograms Blank. I From these data, it is apparent that the presence of the zinc pellets-resulted in significant separation of metallie mercury from the sludge. v

Recovery of the mercury from the zinc-mercury amalwithin approximately one microgram in the 0 to 10 microgram range. For amounts of more than 10 micrograms, the results are significant within about 10% of the reported values. Thus the dataindicate that practically no mercury was recovered from the sedimentcontaining 2S p.p.m. This sediment is thoughttofcontairimercuryjn the form of oxide or sulfide rather-thaujas metallic mercury. The sample containing .l3'l .:p.p.m.-mercury appeared to be capable of releasinga significant fraction of its mercury to the galvanized iron. 1

What is claimed is:

1. A method for separating metallic mercury from sediment at the bottom of a water'body which comprises below about 1 mm. of mercury. After recovery of the mercury by distillation, the zinc is cleaned as by washing with a dilute mineral acid and reused.

EXAMPLE 2 a 'River mud containing a trace amount of mercury was placed in a petri dish. Four coupons of galvanized steel,

22 gauge sheet cut into 3 x 3 cm. squares, were placed on top of the mud and suflicient water was added to completely cover the mud and the metal coupons. After a certain interval of exposure, one coupon was removed and replaced with an identical coupon. The one removed was analyzed to determine the mercury absorbed on its surface. a

The results of such tests on two muds containing about 25 and 131 parts per million by weight mercury, respec- The above data were obtained by dissolving the entire sample in aqua regia, making the solution to volume, and analyzing aliquots for mercury by the so-called cold overlaying the sediment with a layer of metal containing a mercury amalgamating content ofjat least -one of aluminum magnesium and zinc to form a mercury amal gam, retrieving the metal layer from .the water body. and distilling mercury from the amalgam on its. surface. 2. YA method as in claim -1 wherein the layer of metal is provided in the form of chain-linked'she'ets: w? 3. A-method as in claim 1 whereinithe-layer of metal is provided in the form of galvanised iron. 4. A method as in claim 1 wherein overlaying the sediments with the metal layer is achieved by placing a layer of metal directly onto the surface of the bottom sediment. p a

. 5. A method as in claim 1 wherein the layer of inetal contains a mercury.amalgamating-' co1itentof distillation is accomplished by heating the metal at a temperature with the range from about 300m 400 C. v 6. A method as in claims wherein the layer or metal containing 'a mercury amalgamating content of zinc is; in the form of sheets which are substantially uniformly and continuously distributed over the sediment. .at the bottom of the water body.

References Cited- UNITED STATES PATENTS 1,119,377 12/1914 2,266,475 12/1941 3,039,865 6/1962 i 3,572,506 3/1971 Bandyetalfn '210.--170X 3,600,156 8/1971 Clapton et al. x HENRY W. 11, ma /Examine} a 3 Us. 01. x1e f5" -109, 121; 210-65, 

